PROJECT SUMMARY The generation of B cell receptor diversity is mediated by RAG recombinases, and it occurs during B cell development in the bone marrow. This is brought about by stochastic rearrangement of V(D)J gene segments to assemble the B cell receptor. These stochastic rearrangements also lead to the development of B cells that recognize self-antigens. These self-reactive B cell clones are dealt with by either clonal deletion, clonal anergy or RAG recombinase-mediated rescue of self-reactive clones by V gene segment replacement. In this proposal, we present an additional mechanism by which autoreactive pre-B cells are rescued from self-reactivity. We propose that Activation-induced Cytidine Deaminase (AID) plays a role in rescuing self-reactive pre-B cells. AID induces two significant genetic alterations in antigen-activated mature B cells; somatic hypermutation and class switch recombination. The former generates high-affinity B cell mutants that are enticed to enter the memory pool, and the latter facilitates Ig isotype switching from IgM to IgG, IgA or IgE. Interestingly, it has been known for over two decades that a small fraction of immature, bone marrow pre-B cells undergo class switching. This is perplexing because it occurs even in pre-B cells which have only rearranged their Ig heavy chain and are yet to rearrange their light chains. We, based on very preliminary data may have an answer to this puzzle. Our central hypothesis is that immature, self-reactive pre-B cells turn on AID and undergo somatic mutation in order to alter their specificity from self- to non-self-reactivity. The class switching that occurs in these bone marrow pre-B cells is a collateral consequence of AID expression. We will test our central hypothesis in two aims. The proposed work is significant because, upon completion, we will have established a new paradigm to explain the rescue of pre-B cells from self-reactivity. We are well- positioned to carry out the proposed studies as our team with the addition of collaborator Dr. Ignacio Sanz, one of the world?s premier experts on autoimmunity, has the requisite collective expertise to complete the proposed studies successfully.